Field of the Invention
This disclosure relates to antenna based systems and methods for aircraft navigation.
Background
Global Positioning System (GPS) antennas used for navigation on aircraft generate considerable backward radiation which is directed downwards towards the ground. This radiation is primarily caused by what is known as “creeping waves” generated by curved surface diffraction. A portion of the RF energy radiated by the GPS antenna is diffracted around the smooth cylindrical surface of the fuselage of the aircraft. This diffracted energy then propagates or “creeps” around the surface fuselage continuously shedding energy as it propagates until it dies out. It is this radiation that creates the back-lobes in the radiation pattern of the antenna that make these GPS antennas very vulnerable to interference from strong radiating sources located on the ground.
GPS antennas on aircraft can be either jammed or interfered with by a large number of sources. GPS signals are very weak due to their long travel distances from GPS satellites that are located 20,000 kilometers above the earth. Hence they encounter a large amount of “space loss” during their long travel distances. Ground based interference sources are relatively much closer to the GPS antennas on the aircraft and suffer much less path loss; hence they can easily overpower the GPS satellites signals and prevent them from being received.
Some of the antennas that create interfering signals originate from radiating sources located on the ground—the most likely scenario. Other signals can originate from antennas located on the aircraft itself, most likely on the lower surface of the aircraft. These antennas may operate at other frequencies on the aircraft and could be communications antennas, aeronautical radio navigation antennas, radar antennas etc. All of these antennas can be potential sources of RFI (Radio Frequency Interference).
Microstrip “patch” antennas are commonly used for building GPS antennas mounted on aircraft due to their low profile for reducing aerodynamic drag and their low cost and ease of manufacture. Microstrip antennae on aircraft are particularly prone to creating “creeping waves” since they use high dielectric constant substrates that can create creeping waves.
The Federal Aviation Administration (FAA) is currently relying on Global GPS navigation for all commercial aircraft flying in the U.S. These systems also go by the name GNSS (Global Navagation Satellite Systems). The GPS modernization program will soon require GPS antennas located on aircraft to receive the new L5 signals operating between 1.164 GHz to 1.188 MHz with a center frequency at 1.176 GHz. This is in addition to the legacy L1 signal operating at a center frequency of 1.5754 GHz (20 MHz bandwidth).
Since the new L5 signal resides in the Aeronautical Radio Navigation Service (ARNS) band it is particularly susceptible to in-band interference from non GPS signals emitted by several U.S. navigation systems. Most prevalent are aircraft and ground based pulsed DME and TACAN beacons (1.025 to 1.150 GHz), JTIDS and MIDS (0.969 to 1.206 GHz), and ATC/ARNS interrogators, as well as harmonics of other VHF and UHF transmissions from communications antennas.
Several new types of broadband ground planes have recently been proposed to address these issues. These ground planes include Novatel's GNSS-750 hemispherical choke ring ground plane, new types of frequency selective cut-off choke ring ground planes, Electronic Band Gap (EBG) and Artificial Magnetic Conductor (AMC) Ground planes and resistivity tapered ground planes made by the Trimble Corp. The design goals of these approaches is to suppress edge diffraction effects from GPS antennas placed on top of planar metal ground planes. They are not flexible enough to be installed with GPS antennas on top of aircraft with curved, cylindrical shape fuselages. They tend to be large, heavy, expensive, inflexible and not suitable for use in compact, portable systems or on aircraft. Many such designs are also limited by bandwidth and cannot cover the entire GNSS band.